Max is right. Rail and regulator volume, temperature, a few µS difference in pulse width, and other variables will affect the drop. I've never considered that a very reliable indication, and the validity of the test (in my book) is to indicate whether all the injectors are flowing about the same, and not whether they meet any kind of flow standard. The only reliable method for testing that is to pull all the injectors and flow test them in a bench fixture measuring liquid volume over pulse time.
If all your injectors are providing the same drop, they are probably relatively close in flow across the set - or at least close enough for your PCM to work with them. That still doesn't tell us it the injectors are atomizing or streaming, what the hot performance might be, whether they are flowing at a design volume, or anything else.